How Worried Should We Be About Radiation From Japan's Nuclear Plant?

As the crisis at the Fukushima nuclear plant in Japan becomes more and more serious, so do worries about radioactive material leaking from the reactors. PM explains what the reactors are releasing now, and how bad the situation could become.

The radiation leaks at the Fukushima Dai-ichi nuclear plant have gone from bad to worse. First workers vented small amounts of radioactive gas to try to cool the reactors in danger of going into meltdown. Then, on March 15, radiation levels shot up to 1000 times higher than previously recorded after an explosion at a fourth facility housing spent fuel rods. On March 16, according to the AP, plant operators briefly pulled the 50 remaining workers out of Fukushima after radiation levels suddenly spiked again, and then ordered them back in to resume their attempts to stave off an even larger disaster. But just how bad could radiation leaks become?

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"There has been a significant amount of radioactivity released into the atmosphere," says Ian Hutchinson, professor of nuclear science and engineering at the Massachusetts Institute of Technology. He says it's more than was released at the Three Mile Island incident in 1979, but less than Chernobyl in 1986. "But where that goes depends on prevailing winds and weather," he explains.

Indeed, it's not just how much radiation that matters, but also what kind, how far it travels, and how long it persists. In the Chernobyl accident, radioactive material exploded into the air. "The plume went across many countries, traveling at 60,000 feet," Kelly Classic of the Health Physics Society says. About 5 percent of the reactor core actually went up into the air and was carried along the jet stream, allowing the radioactive material to spread across Europe.

As PM noted this week, though, Fukushima—however badly it turns out—is not Chernobyl. Despite disaster at multiple reactors, Fukushima uses secondary containment systems Chernobyl lacked that have prevented core fuel from escaping the reactor. "It's not so much the meltdown itself," Classic says, "it's whether or not it breaches the reactor vessel and gets into the soil, or a build up of pressure and heat causes an explosion to propel fuel into small pieces out into the atmosphere."

So far, the explosions at Fukushima have not been great enough to skyrocket material to the heights observed during Chernobyl. That's because Fukushima uses water as a coolant and as a way to slow down neutrons unleashed by the fission process, thereby slowing the process itself. The Chernobyl plant was designed to use graphite to control the nuclear reaction instead, and when the accident happened, the graphite caught fire. "When the graphite burned, it was like a campfire, and carried away parts of the nuclear fuel rods," explains Barry Brook, professor of environmental sciences at the University of Adelaide.

That hasn't happened in Japan, but there are signs that Fukushima fuel pellets have melted. Melting of the pellets alone is not enough to project fuel into the atmosphere, but it will release more radioactive fission products. If the containment vessel is breached as a result of melting, radioactive fuel could leak into the ground, or pressure buildup could potentially cause an explosion that projects radioactive material into the air. The Nuclear Energy Institute and others worry that containment leaks could be happening at Fukushima reactors 2 or 3, but Japanese officials are providing few details on the severity of any leaks.

The biggest radioactive risk right now comes from the byproducts of fission. Tokyo Electric Power Company, which runs the reactors, has reported releases of both iodine-131 and cesium-137, the two primary radionuclides that nuclear fission creates. According to Hutchinson, strontium-90 has also been detected, and the presence of cesium and strontium indicates fuel melting.

Iodine-131 moves through the atmosphere more easily than cesium-137, but it has a half-life of only eight days, according to Classic. That means it would be all but gone within weeks. Cesium-137, on the other hand, attaches itself to particles or debris. That means that eventually cesium-137 will fall out of the air onto the ground, and there it will stay until it decays. The isotope's half-life is about 30 years, so it would be a long time before an area it traveled to would be free from radiation. Depending on the level of radiation, the area would have to be sectioned off or the material dealt with by a hazardous waste disposal team. According to the Environmental Protection Agency, exposure to radiation from cesium-137 near a nuclear accident site could significantly increase the risk of cancer. Trace amounts of cesium-137 are already in the environment worldwide, mostly because of nuclear weapons testing in the 1950s and '60s, but most of that has decayed.

It could be weeks until scientists know for sure exactly what types of radioactive nuclides have been released and where they have gone. So far, Classic says, people outside the evacuation zone should not have been exposed to serious health hazards. However, low but abnormal levels of radiation have been detected. The Department of Defense issued a statement Monday that a Navy vessel 100 miles from Fukushima detected contamination equivalent to about a month of natural radiation exposure.

But for now, the main risk to human health is for those still inside the evacuation zone, within a 20-mile radius from the plant (though the U.S. embassy is now recommending Americans to stay at least 50 miles away from the plant). That's especially true for the skeleton crew that remains at the plant to try to fight off the worst-case scenario. Even without a complete nuclear meltdown, the situation is dire near the reactors, Classic says: "People exposed to that level for many hours are going to have some health effects." As of March 16, the reading around Fukushima reactor 1 was 3,391 microsieverts, or 339 millirem. That is near the average amount of radiation exposure the average person gets in an entire year, according to the EPA.